A conventional method of making low loss multicomponent glass fiber involves drawing fiber from a double crucible whose inner crucible is charged with core glass and whose outer crucible is charged with cladding glass. Contamination of the melts is usually the factor that limits the ultimate transmissivity of fiber produced by this method. This method is however not suitable for the manufacture of fused silica fiber on account of the high melting point of silica. For the manufacture of fused silica fiber an alternative method has been disclosed involving the chemical vapor deposition of a doped silica upon the bore of a fused silica tube. This method has the advantage that the core glass is prepared in situ, and can be prepared in particularly pure form. However it has the disadvantage that the numerical aperture and the core to cladding diameter ratio of the resulting fiber is restricted in comparison with that of multicomponent glass fiber obtainable with the double bushing method. This is primarily because the doping concentration for the deposited layer, in relation to the thickness of that layer, is restricted in order to prevent too great a mismatch in thermal expansion between the doped material of the deposited layer and the underlying fused silica tube. Pure fused silica has a particularly low thermal expansion compared with most other glass systems, and too large a mismatch causes cracks.